1. Field of the Invention
The present invention relates to a four-wheel-driving system for varying the driving power distributed between front and rear wheels of a motor vehicle.
2. Prior Art
In order to allow a four-wheel-drive utility vehicle to make full use of its controllability and driving power, the driving power of a vehicle has, in the past, been distributed to the front and rear wheels in response to the road surface and the driving conditions. For example, the Published Japanese Patent Application sho No. 59-151661 discloses that a center differential, which distributes driving power between the front and rear wheels, is combined with a continuously variable transmission to obtain a predetermined distribution ratio of the driving power. In this case, differential gear devices are provided at the front and rear wheels, and further at center. Therefore, the output axle of the transmission rotates much faster than usual, thereby increasing noise and vibration. Since the center differential comprises additional gearing, the endurance thereof is liable to be reduced by the increased vibration.